In time-of-flight (TOF) depth imaging, a subject is irradiated by pulsed or modulated light from an emitter. Some of this light reflects back from the subject to an imaging photodetector. The pixel elements of the photodetector are addressed in groups, in synchronicity with the pulsed output of the emitter. In some variants, the integration periods for each group of pixel elements differ by a predetermined amount. Accordingly, a pixel-resolved time of flight of the pulses, from their origin at the emitter, out to the subject, and back to the photodetector, is discernible based on the relative amounts of light received in corresponding elements of the different groups. From the time of flight computed for a given pixel, the depth of the locus of the subject imaged by that pixel—i.e., the distance away from the photodetector—can be computed.
In the approach summarized above, the signal-to-noise ratio for depth measurement is reduced in the presence of broadband ambient light. In principle, the signal-to-noise ratio can be improved by increasing the output power of the emitter, such that the reflected light from the emitter overwhelms the ambient light. However, increasing emitter power may also increase the size, complexity, and cost of the imaging system.